clojure.core/update

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Type: function Added: Clojure 1.7 Examples: 10 Runnable: 6

([m k f] [m k f x] [m k f x y] [m k f x y z] [m k f x y z & more])

'Updates' a value in an associative structure, where k is a key and f is a function that will take the old value and any supplied args and return the new value, and returns a new structure. If the key does not exist, nil is passed as the old value.

Examples

by hiteki on 2015-07-17 ▶

(def p {:name "James" :age 26}) ;;=> #'user/p (update p :age inc) ;;=> {:name "James", :age 27} ;; remember, the value of p hasn't changed because it is immutable! p ;; {:name "James", :age 26} (update p :age + 10) ;;=> {:name "James", :age 36} ;; Here we see that the keyed object is ;; the first argument in the function call. ;; i.e. :age (- 26 10) => 16 (update p :age - 10) ;;=> {:name "James", :age 16}

by egracer on 2015-11-25 . May have evaluation errors. ▶

;; the map in update can be nil, and f will still be applied to nil and ;; return a value (def empty-map nil) #'user/empty-map (update empty-map :some-key #(str "foo" %)) ;;=> {:some-key "foo"}

by G1enY0ung on 2016-10-19 ▶

;; can also use in [] (update [1 2 3] 0 inc) ;;=> [2 2 3] (update [] 0 #(str "foo" %)) ;;=> ["foo"]

by statcompute on 2018-04-22 ▶

(def ds [{:id 1.0 :name "name1"} {:id 2.0 :name "name2"} {:id 3.0 :name "name3"}]) (map (fn [x] (update x :name #(if (= "name2" %) % "not 2"))) ds) ;; ({:id 1.0, :name "not 2"} {:id 2.0, :name "name2"} {:id 3.0, :name "not 2"})

by aarkerio on 2018-08-31 ▶

;; From string to boolean (def answer {:answer "France" :correct "true" :age 11}) (update answer :correct #(= % "true")) ;; {:answer "France", :correct true, :age 11}

by gloorfindel on 2020-09-27 . May have evaluation errors. ▶

;; Applying a function to the same key in a sequence of arrays (def dates [{:date "20200430",:time "08:35"} {:date "20200430",:time "09:15"}] ;; Takes the current time value in the array, and reduces it to the hour by ;; calling the reduce-to-hour function (not included here) (map #(update % :time (constantly (reduce-to-hour (:time %)))) dates) => ({:date "20200430", :time "08"} {:date "20200430", :time "09"})

by huahaiy on 2021-02-13 ▶

;; Very common, update a vector inside a map (def little-schemer {:authors [{:birth-year 1944, :name "Daniel Friedman"} {:name "Matthias Felleisen"}] :title "The Little Schemer"}) (update little-schemer :authors conj {:name "Gerald J. Sussman"}) ;;=> {:authors [{:birth-year 1944, :name "Daniel Friedman"} {:name "Matthias Felleisen"} {:name "Gerald J. Sussman"}] :title "The Little Schemer"}

by rogefm on 2021-02-23 ▶

;; For a simple change of value we can use constantly (def hm {:banana 1, :apple 2, :orange 3}) (update hm :banana (constantly 6)) ;;=> {:banana 6 :apple 2 :orange 3} ;; Which is equivalent to using "assoc" (assoc hm :banana 6) ;;=> {:banana 6 :apple 2 :orange 3}

by mars0i on 2021-06-16 . May have evaluation errors. ▶

;; A quirk in use with vectors (tested with 1.10.3) ;; If you throw away the argument to the function, you can add a value to ;; the end of the vector: ;; Normal, expected behavior: (update [1 2 3] 2 (fn [_] 42)) ;;=> [1 2 42] ;; If the index is just past the end, you can append to the vector: (update [1 2 3] 3 (fn [_] 42)) ;;=> [1 2 3 42] ;; More than one past end won't work (update [1 2 3] 4 (fn [_] 42)) ;;=> Execution error (IndexOutOfBoundsException) at user/eval2049 (form-init3979469036338260438.clj:1). ;; If you don't throw away the argument, and your index is one past ;; the end of the vector, you'll still get an error: ;; Normal, expected behavior: (update [1 2 3] 2 inc) ;;=> [1 2 4] ;; Here the trick fails: (update [1 2 3] 3 inc) ;;=> Execution error (NullPointerException) at user/eval1985 (form-init8618353961027343980.clj:1).

by lpasz on 2024-11-07 . May have evaluation errors. ▶

;; Will update the value by incrementing it (update {:a 1} :a (fn [value] (inc value))) ;; => {:a 2} ;; Will raise because there is no key with that value (update {} :a (fn [value] (inc value))) ;; => Execution error (NullPointerException) at user/eval1 (REPL:1). ;; Cannot invoke "Object.getClass()" because "x" is null ;; In some languages update would provide a default value if none is found ;; Map.update(map, key, default_value, function) ;; Map.update(%{}, :a , 1 , & &1 + 1) => %{a: 1} ;; But in Clojure, if no value is found nil is returned to the update function. ;; So to fix that function we would need to (update {} :a (fn [value] (if value (inc value) 1))) ;; => {:a 1} ;; Other arguments at update level will just be passed along to your function (def dollar-rate 5.70) (defn add-funds-to-account [my-account amount coin] (update my-account :amount-in-brl (fn [curr-amount added-amount coin] (case coin :brl (+ added-amount curr-amount) :usd (+ (* dollar-rate added-amount) curr-amount))) amount coin)) (add-funds-to-account {:amount-in-brl 10} 10 :brl) ;; => {:amount-in-brl 20} (add-funds-to-account {:amount-in-brl 10} 10 :usd) ;; => {:amount-in-brl 67.0}

Note by alper

But how do you do a pure update idiomatically? So update one value for another?

Note by Crowbrammer

To the previous note (alper), you are looking for assoc.

Note by earthfail

in clojure 1.11 there is an update-vals and update-keys to apply function for all values or all keys respectively

See also

• clojure.core/update-in (10)
• clojure.core/fnil (1)
• clojure.core/assoc (8)
• clojure.core/associative?

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